You're spoilt for choice on Ebay, with so many different makes and models of flash available that it's difficult to choose which one is right for you - this guide should make it a bit easier.
Basically all flash heads work in the same way.
When you press the magic button on your camera the shutter opens just long enough for the flash to fire, then it closes again. The flash might be connected directly to your camera by an electrical cord (the old fashioned way), it might be fired by an infra red signal from a small transmitter fitted to your camera hotshoe or it might be fired by a radio trigger, again fitted to your hotshoe (the best and most reliable method) - but whichever method you use, the flash unit itself works in the same way.
So if they all work in the same way, why does it matter which one you buy?
- Well, what the flash actually does is to provide a very brief, intense flash of light. That's fine, but the light usually needs to be changed (modified) in some way. For example, you might want to fit an umbrella or softbox to the light, effectively this makes the light larger and so the light it produces is softer. Or you might want to go in the opposite direction and fit a honeycomb or a spotlight attachment. Or you might just want to use a different size or shape of reflector. The better lights have removable reflectors so that softboxes and other accessories can be fitted, and anyway it's a very good idea to make sure that you can change the reflector on your flash. You probably won't be happy with the results from flash heads that have fixed or built-in reflectors, and you won't get good results from them if you use them with a softbox because the light won't spread as well if the reflector can't be removed.
- And even if the reflector can be removed, your flash head needs to have a fitting that can take a wide range of accessories. Elinchrom and Bowens are the leading brands and although there is no 'standard' fitting some manufacturers design their lights to take Elinchrom or Bowens accessories, but other manufactuers have their own fitting and you won't be able to get a range of accessories.
- And then there's the question of power. You may not need a lot of power but quite frankly some of the flash kits sold on Ebay have such low power that they may not do the job you want them to - especially with accessories fitted, which can reduce the amount of light to a quarter.
- And then there's power adjustment. You need to be able to turn the power down when you need to, and some flash units don't adjust at all, or only adjust by a very small amount (say to a 1/2 or a quarter of full power). If there isn't enough adjustment you will need to fit neutral density gels to reduce the power, it's far better to get lights that adjust over about 5 stops (32:1)
- The modelling lamp is important too. Because flash heads only fire for a fraction of a second you need to have a continuous light too, so that you can get an idea of the effect that the flash is going to have. The best lights use a quartz halogen lamp of at least 250 watts for this, some of the cheaper ones have modelling lamps that are so dim that they're totally useless.
- Another thing to think about is recycling time - that's the time it takes the flash to get ready for the next shot. Slow recycling times don't matter if you're photographing still life but if you're photographing people you'll get pretty frustrated if the flash hasn't recharged by the time you want to take the next picture. Some flashes recharge in less than a second, but some take much longer.
- Of course, build quality may be important too. Some of the flash heads built down to a price may not be built very sturdily.
There's a lot of very detailed info in my tutorial about studio lighting and equipment
I hope this helps you to make your choice. Not everyone will find that this guide helps them - for example the people who sell the type of equipment I'm advising against may find it unhelpful because people will make better choices, so please be sure to let other Ebay users know your opinion by voting. The more votes it gets, the more visible the guide becomes and the more people will benefit from it.